when someone cheats in a relationship, is the blame on the cheater, the cheated, or both partners?

If your answer assumes a gender for either partner, please state that, because I'm aware that standards are different for different genders, and for different sexual orientations.

I have heard too many arguments and I'm confused now. Some say it's on the cheater, because they should resolve issues or just leave. Some say it's on the cheated, because they did not provide what their partner needed, and the cheater cannot be blamed for looking for it outside of the relationship. The rest tell me that it's on both partners, because they let communication between them break down, and let themselves drift apart.

I'd really like to have a substantiated opinion on this before I get into a relationship, in case I get cheated upon.

For more context, I'm young and new to dating, and the scene itself scares me.